Tornadoes are the most dangerous storms that nature creates. There are different levels of severity that a tornado can have. Some tornadoes just have strong winds but other levels can be deadly.
A tornado is rapidly rotating column of air descending from a large thunderstorm. For a tornado to occur from an ordinary thunderstorm several conditions must occur simultaneously: warm moist air from the Gulf of Mexico, dry cold air moving down from Canada or the Rocky Mountains, and jet stream winds racing east. These three air masses all moving in different directions collide into each other. Warm air rises by convection to form a vertical cloud mass. Cooling condenses water vapor in the air into the rain falls back down through the cloud cooling the lower air. Wind shear can tilt the thundercloud mass, causing rotation of the spinning roll of air.
Although the air is rising in a tornado, the funnel itself grows from the cloud toward the ground as the tornado is forming. The term "funnel cloud" refers to a tornado-like vortex that doesn't reach the ground. When a funnel cloud touches the ground, it becomes a tornado.
Tornadoes are also relatively small. An average tornado will be 400 to 500 feet wide and travel four or five miles on the ground, lasting only a few minutes. A mile-wide tornado is an extremely large one and tornadoes this big is rare. Many tornadoes are small, less than 100 feet wide, and last only a few minutes.
Some states are in a place called Tornado Alley. This is an area of the United States that is more likely than other places to have tornadoes. While other states can have a tornado, these states are the most likely to have them on a regular basis: Arkansas, Iowa, Kansas, Louisiana, Minnesota, Nebraska, North Dakota, Ohio, Oklahoma, South Dakota, and Texas.
In order to understand how dangerous a tornado is, weather specialists created a scale to measure how much possible damage is expected. This scale looks at the speed that the tornado travels and the size of the tornado. The scale is known as the Fujita scale. The scale moves from 0 to 5. Zero is the least amount of damage and is sometimes referred to as funnel clouds. There is not much damage expected with these. The highest level is the F5 which is the most destructive type of tornado.